In this respect, the document U.S. Pat. No. 7,889,524 B2 is an integrated bidirectional converter for hybrid vehicles with electrical charging, said converter comprising a single induction coil as an inductive store and enabling an electrical charging function via an AC/DC converter. Furthermore, in a so-called boost function, by means of the bidirectional DC/DC converter, power of a rechargeable battery can be supplied to a first vehicle onboard power supply system with a higher operating voltage than the rechargeable battery and, conversely, in a so-called buck function, it is possible to output power from the first vehicle onboard power supply system to the rechargeable battery in a second vehicle onboard power supply system with the operating voltage of the rechargeable battery. For this purpose, the vehicle onboard power supply systems coupled via the DC/DC converter comprise a control device.
However, the state of a rechargeable battery in such vehicle onboard power supply systems is neither indicated nor checked in the vehicle. By contrast, in conventional vehicles having a 12 V onboard power supply system and internal combustion engine, a test of a 12 V rechargeable battery implicitly takes place during every process of starting the internal combustion engine. In this case, a high current is drawn from the rechargeable battery during the starting process. If the starting process is then possible, this is associated with a check of the minimum state of charge, also called SOC state, and of the aging state, also designated as SOH state (State of Health). If the starting process is not possible, either the minimum SOC state or the minimum SOH state of the rechargeable battery cannot be provided, with the result that the vehicle cannot be started up.
This means that in conventional vehicles a safety risk as a result of an inadequately functioning battery cannot occur either. However, in hybrid vehicles or electric vehicles, which usually comprise vehicle onboard power supply systems having different operating voltages, such as a high-voltage side and a low-voltage side, and which do not have a starter on the low-voltage side, this starting process test and thus the conventional test on the low-voltage side is omitted.
However, since a minimum SOC state and a minimum SOH state of the low-voltage rechargeable battery are required for safe operation of these vehicles, too, a corresponding check must be carried out recurrently in these vehicles, too. This check of the rechargeable battery is carried out with the aid of vehicle-external checking devices at correspondingly prescribed maintenance intervals. However, if the registered keeper of the vehicle does not adhere to said intervals, then such vehicles constitute a safety risk which can only be reduced by a timely warning of the driver of the vehicle.